1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display which has storage capacitance cells formed on scanning lines, and which displays one color by combining a plurality of fundamental colors, e.g., red, green and blue if it is a multicolor display.
2. Description of the Related Art
An active matrix type liquid crystal display has a pair of substrates between which a liquid crystal is maintained, a plurality of scanning lines and a plurality of signal lines arranged so as to form a matrix, a switching device such as a thin-film transistor (TFT) provided in each of regions defined between the scanning lines and the signal lines, and a pixel electrode connected to each switching transistor.
FIG. 14 shows arrangement of components constructed on one of regions defined between scanning and signal lines on one of a pair of substrates for an active matrix type liquid crystal display, i.e., a thin-film transistor array substrate. Conductors which extend laterally as viewed in FIG. 14 are scanning lines G.sub.1 and G2 while conductors which extend vertically are signal lines S.sub.1 and S.sub.2. A semiconductor active film 1 is formed on the scanning line G.sub.1 with a gate insulating film interposed therebetween. A source electrode 2 extending from the signal line S.sub.1 and a drain electrode 3 are formed on the semiconductor active film 1. The scanning line G.sub.1, the source electrode 2, the drain electrode 3 constitute an inverted staggered thin-film transistor T. A pixel electrode 4 is formed on a passivation film which covers the source electrode 2 and the drain electrode 3. The pixel electrode 4 which is formed over the region defined between the scanning lines G.sub.1 and G.sub.2 and between the signal lines S.sub.1 and S.sub.2 has an extension formed over a portion of the preceding-stage scanning line G.sub.2 for driving the adjacent pixel electrode 4. The pixel electrode 4 over this region is electrically connected to the drain electrode 3 via a contact hole 5, and forms a storage capacitance cell C.sub.0 in association with the scanning line G.sub.2 through the gate insulating film and the passivation film.
On the other substrate opposed to the thin-film transistor array substrate are provided a common electrode and a black matrix for stopping light at regions where the thin-film transistor T, the scanning lines G.sub.1 and G.sub.2, the signal lines S.sub.1 and S.sub.2 are provided. The black matrix has a rectangular aperture 6 indicated by the dot-dash line in FIG. 14. Light can pass through the aperture 6. The aperture 6 corresponds to an effective display area which contributes essentially to display.
If the liquid crystal display is a monochromatic display, each region defined between the scanning lines G.sub.1 and G.sub.2 and the signal lines S.sub.1 and S.sub.2 corresponds to one pixel. If the liquid crystal display is a multicolor display, a color filter having different fundamental colors, e.g., red, green and blue is provided on the opposed substrate, each color corresponding to one of the regions defined between the scanning lines G.sub.1 and G.sub.2 and the signal lines S.sub.1 and S.sub.2. One region corresponds to one dot, and a set of three dots is formed as one pixel.
It is necessary for the above-described conventional liquid crystal display to be designed so that all peripheral portions of each pixel electrode 4 overlap the black matrix. This is because there is a possibility of a disturbance of the orientation of liquid crystal molecules due to a disturbance of the electric field caused by interference with the electric fields formed by the adjacent pixel electrodes, resulting in occurrence of a leak of light at the peripheral portions of the pixel electrode 4. It is necessary to stop such a leak of light by the black matrix. The width W of each of the overlap portions is a value determined as the sum of a width necessary and sufficient for stopping a light leak and an overlap margin selected by considering the accuracy of superposition of the thin-film transistor array substrate and the opposed substrate. On the other hand, the area of the overlap portions is a factor of restriction the aperture ratio, and there is a need to minimize the area of the overlap portions in order to increase the aperture ratio.
In the case where the pixel electrode 4 is formed so as to extend over a portion of the scanning line G.sub.2 to form a storage capacitance cell C.sub.0 on the scanning line G.sub.2 as shown in FIG. 14, the scanning line G.sub.2 functions as a black matrix portion because of the overlap of the pixel electrode 4 and the scanning line G.sub.2. In other words, the aperture ratio is not changed by an extension of the aperture 6 of the black matrix on the scanning line G.sub.2 side, and the effective aperture ratio is determined by the size of the portions of the pixel electrode 4 and the scanning line G.sub.2 superposed on each other. If the area of the portion of the pixel electrode 4 where light is stopped by the portion of the black matrix superposed on the pixel electrode portion is S; the width of the aperture along the scanning line direction (the distance between edges of the black matrix) is x; and the width of the aperture along the signal line direction (the distance between an edge of the black matrix and the scanning line) is y, the light stop area S is roughly expressed by the following equation: EQU S=W.times.(x+2y) (1)
Thus, the influence of the width y along the signal line direction is twice as large as that of the width x along the scanning line direction. That is, an increase in S when y is increased by a certain amount is larger than an increase in S when x is increased by the same amount.
In conventional liquid crystal displays, however, the pitch between adjacent signal lines and the pitch between adjacent scanning lines are such that, if the former pitch is X and the latter is Y, Y is generally about thrice X, for example, X=100 .mu.m and Y=300 .mu.m. Naturally x&lt;y, since X&lt;Y (3X=Y). The increase in the light stop area by an increase in the width y of the aperture along the signal line direction is larger, so that the increase in the aperture ratio is limited.